Ai Weiwei has said that police in Beijing are preventing him from attending a scheduled court hearing regarding his ongoing tax case with the Chinese government. The artist told reporters this week that members of the police have positioned themselves outside his Beijing studio to prevent his planned appearance at the hearing.

On Twitter, Ai said Wednesday that his associate, Liu Xiaoyuan, who is a lawyer and blogger, has been taken forcibly into police custody.

Ai is contesting a $2.4-million tax bill imposed by Chinese officials. The artist was imprisoned for 81 days in Beijing last year and was released on bail. Ai has been an outspoken free-speech advocate and has used Twitter and other social media outlets to repeatedly criticize the Chinese government.

On Twitter, Ai said Wednesday that his wife, Lu Qing, attended a court session in his place.

In May, the Chinese government reversed its decision to deny Ai the right to challenge the tax bill in court. The tax bill is being levied against Ai's company. The artist has claimed that the Beijing tax office violated the law in its handling of witnesses, evidence and more.